Rocks come in a wide variety of colors, including green. The green color in rocks is often caused by the presence of minerals containing elements like iron, nickel, copper and chromium. Some of the most common green rocks include serpentine, chlorite schist, epidote, malachite and amazonite.
Serpentine
Serpentine is a group of common hydrous magnesium iron phyllosilicate minerals. It is mostly commonly green in color, with shades ranging from light yellowish-green to very dark green. The green color comes mainly from iron. Serpentine has a greasy or waxy luster and a hardness of 2.5 to 4 on the Mohs scale.
Serpentine forms by the metamorphic alteration of ultramafic rocks, often in areas of low temperature or pressure. It is found in many locations around the world, including Italy, Russia, New Zealand, Canada and the USA. Serpentine has been used as a decorative stone and carved into art objects and jewelry for thousands of years.
Chlorite Schist
Chlorite schist is a type of metamorphic rock made up mainly of chlorite, talc, and quartz. It often has a distinct greenish color caused by the chlorite minerals present. Chlorite schist forms when shale comes into contact with hydrothermal fluids or heat and pressure, altering it into a metamorphic rock.
The platy, flaky structure of the original shale is still visible in the schist. The green chlorite minerals grow in between the flaky layers, giving it a foliated texture. Chlorite schist is typically soft, with a hardness of 1 to 2 on the Mohs scale. It can be found in many metamorphic terrains around the world.
Epidote
Epidote is a family of aluminum iron calcium silicate minerals. It commonly forms prismatic, columnar or granular crystals. Epidote has a hardness of 6 to 7 on the Mohs scale. Its color is usually yellowish-green to pistachio green, but can also range to colorless, grey and black.
The vivid green color comes from iron. Epidote forms in metamorphic and igneous rocks rich in calcium and aluminum. It is also found in hydrothermally altered rocks. Notable occurrences include Austria, Brazil, Madagascar, Pakistan, Russia and the United States. Epidote has been used as a gemstone and as a source of iron ore.
Malachite
Malachite is a bright green copper carbonate hydroxide mineral with a hardness of 3.5 to 4. It commonly forms botryoidal masses with concentric color banding. The striking green color results from its high copper content.
Malachite forms through the weathering of copper ores. Notable deposits occur in the Ural Mountains in Russia, the Democratic Republic of Congo, and in Arizona and Nevada in the USA. It has been used as a gemstone, pigment and ore of copper since ancient times.
Amazonite
Amazonite is a green variety of microcline feldspar. It gets its name from the Amazon River in Brazil, although no deposits are known from there. Amazonite ranges in color from light green to deep blue-green. It has a Mohs hardness of 6 to 6.5.
The green color results from lead impurities within the feldspar structure. Amazonite forms in granitic pegmatites and is also found in metamorphic rocks. Notable occurrences include Colorado and Virginia in the USA, Madagascar and Russia. Amazonite has been used as a gemstone and ornamental stone for thousands of years.
Other Green Rocks
In addition to those already mentioned, some other less common green rocks include:
- Glauconite – An iron potassium mica mineral that gives a greenish cast to sedimentary rocks.
- Celadonite – A mica group mineral ranging from gray-green to dark green in color.
- Prasiolite – A green variety of quartz sometimes used as a gemstone.
- Verdite – A cryptocrystalline massive variety of serpentine.
- Greenstone – A metamorphosed basic igneous rock that owes its color to chlorite, epidote and other green minerals.
Causes of Green Color in Rocks
There are several common factors that can cause a green color in rocks and minerals:
- Presence of iron – Iron can impart a greenish color in minerals like serpentine, chlorite, epidote and glauconite.
- Weathering of copper – Malachite and other copper-bearing minerals turn green when copper weathers and oxidizes.
- Lead impurities – Trace amounts of lead in minerals like feldspar cause the green color in amazonite.
- Hydrous minerals – The presence of water in mineral structures like serpentine influences the green color.
- Metamorphism – Green minerals form when basic igneous rocks metamorphose into greenstone.
The specific shade of green depends on the particular mineral composition and geology of the rock. Identifying the exact green minerals present can help determine the processes that formed the rock.
Geological Significance
The presence of green rocks can provide useful information about the geological history and mineral content of a region:
- Greenserpentine often forms in areas with ultramafic source rocks.
- Green schists indicate regional metamorphism of shales.
- Malachite points to weathering of copper deposits.
- Amazonite indicates the presence of granitic pegmatites.
- Glauconite helps identify sedimentary marine environments.
Analyzing the distribution and alignments of green rock units can trace structural features like faults and shear zones. The mineral assemblages in green rocks also indicate the temperatures, pressures and chemical environments involved in their formation.
Uses
Some green rocks have served various uses throughout human history, including:
- Gemstones – Malachite, amazonite, and some serpentines are used as ornamental and gem stones.
- Construction – Serpentinite has been used as a building stone.
- Decoration – The beauty of malachite and serpentine has led to their use in art, jewelry, and ornamental objects.
- Industry – Serpentine is used to make asbestos and talc. Epidote and malachite are sources of iron and copper ore respectively.
Some varieties like serpentinite and chlorite schist are less durable and unsuitable for most uses. Collectively, green rocks constitute a small but mineralogically and historically significant portion of the rocks on Earth.
Summary
Green coloration in rocks often results from the presence of iron, copper, and other metal ions in chlorite, epidote, serpentine and other hydrous silicate minerals. Greenish rocks form in a variety of geological environments including regional metamorphism, hydrothermal alteration, and weathering of mafic and ultramafic source rocks. The green minerals provide clues about the chemical and physical processes that formed the rocks and can indicate the presence of economically useful metal deposits. Some green rocks have served as gemstones and decorative and building materials since ancient times.
| Mineral | Chemical Formula | Major Element(s) |
|---|---|---|
| Malachite | Cu2CO3(OH)2 | Copper |
| Serpentine | (Mg,Fe)3Si2O5(OH)4 | Magnesium, Iron |
| Chlorite | (Mg,Fe)3(Si,Al)4O10(OH)2-(Mg,Fe)3(OH)6 | Magnesium, Iron |
| Epidote | Ca2Al2FeSi3O12(OH) | Calcium, Iron |
| Amazonite | KAlSi3O8 | Potassium |
Conclusion
Green rocks contain a range of minerals, but many owe their coloration to iron, copper and other metal ions that impart greenish hues. Understanding what causes the green colors provides geologists with insights into the geological processes and chemical environments that formed and altered the rocks. Identifying the specific green minerals and their chemistry helps decode the story told by these colorful and interesting rocks.
